Conventionally, scattered light methods are used for measuring the concentration of particles in exhaust gases and other colloids.
In this context, an intense light source, such as a laser, situated in a measurement chamber, is usually used, and the colloid to be measured is guided through the measurement chamber. In the measuring chamber, there is at least one light sensor which detects scattered light, that has been scattered by the particulates present in the colloid. Even when so-called purge air curtains are used in order to keep the light output areas of the light sources and the light input areas of the light sensors, that come into contact with the exhaust gases, free of deposits, the light input areas and the light output areas become soiled, in the course of time, by deposits of particulates, the intensity of the signals emitted by the light sensors decreases and the measuring results are corrupted. A corruption of measuring results may also be caused by the aging of components of the light source, of the light sensors and/or the amplification electronics and evaluation electronics.
It is therefore important to detect soling and/or aging of the critical components of such a meter, in order to assess the accuracy and the validity of the data measured by using the meter.
Exhaust gas measuring instruments, that are used for official measurements, are covered by official calibration duty, whereby the necessity for assuring sufficient accuracy of the measurement results increases even further.